Bond-rearrangement during molecular dissociation leads to the formation of
molecules from atoms not directly bound in the precursor molecule. One such
process is the decay of the transient H2O+ ion into
H2+ + O, which requires
the formation of a H-H bond during dissociation. It is important to determine
if the new bond is formed during the slow dissociation or if the system makes
a sudden transition between the initial and final bound states. Intuitively one
would expect heavier isotopes to dissociate slower, and thus the bonds would
rearrange easier if the first mechanism dominates. We measured the hydrogen
molecular ion formation rate from H2O, HDO and D2O bombarded
by fast ions,
and found that it depends strongly on the fragment mass. This suggests that the
rearrangement does not happen during the slow dissociation, but rather during
the very fast ionization. Calculations are underway to determine the relative
production rates for the different isotopes from the overlap of the initial and
final vibrational wave functions. Preliminary measurements indicate that the
transient H_2O^2+ ion decays also to H2+ + O+;
the question is if it shows a
similar isotopic dependence as predicted by sudden bond rearrangement.

This work was supported by the
Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences,
Office of Science,
U.S. Department of Energy.